1. Field of the Invention
The present invention relates to a charging circuit capable of efficiently performing charging by detecting the direction of current from a voltage difference between two different power supplies.
2. Description of the Related Art
A charging circuit 6 having two different power supplies, a storage cell 2 and a photogenerating cell 30, and a backflow preventing diode 40, similar to the one shown in FIG. 6, has been known in the past.
The charging circuit 6 uses the storage cell 2 thereof to drive a drive circuit 5. When a voltage produced at the photogenerating cell 30 is higher than the one at the storage cell 2, the photogenerating cell 30 can charge the storage cell 2. The photogenerating cell 30 has a positive electrode having a reference potential 1 of the charging circuit 6, and a negative electrode having a power supply potential of the charging circuit 6.
The photogenerating cell 30 adopts the structure of a pn junction having a p-type semiconductor and an n-type semiconductor joined together. Specifically, four pn junctions are connected in series with one another in order to exert an electromotive force of approximately 2.8 V.
The backflow preventing diode 40 is connected between the storage cell 2 and photogenerating cell 30 so that the direction of current that flows from the photogenerating cell 30 to the storage cell 2 will correspond to the forward direction of the backflow preventing diode.
Moreover, the drive circuit 5 driven by the charging circuit 6 is connected between the positive electrode (reference potential 1) and the negative electrode (supply potential).
Next, the actions of the charging circuit 6 shown in FIG. 6 will be described below.
To begin with, a description will be made of a case where the voltage at the storage cell 2 is lower than the one at the photogenerating cell 30.
A reverse current produced by the photogenerating cell 30 serves as the charging current for the storage cell 2. The direction of the current corresponds to the forward direction of the backflow preventing diode 40. The flow of the current is therefore not prevented but the storage cell 2 is charged. Incidentally, the forward voltage developed at the backflow preventing diode 40, through which the current flows in the forward direction, is approximately 0.4 V. Therefore, unless the voltage difference between the photogenerating cell 30 and storage cell 2 is equal to or larger than 0.4V, charging cannot be achieved in practice.
Next, a description will be made of a case where the voltage at the storage cell 2 is equal to or higher than the one at the photogenerating cell 30.
When the voltage at the storage cell 2 is equal to the one at the photogenerating cell 30, the voltages are balanced. No reverse current therefore flows from the photogenerating cell 30. When the voltage at the storage cell 2 is higher than the one at the photogenerating cell 30, current attempts to flow from the storage cell 2 to the photogenerating cell 30. However, as the direction of the current corresponds to the reverse direction of the backflow preventing diode 40, the flow of the current to the storage cell 2 is blocked.
Moreover, the backflow preventing diode 40 is realized with a metal-oxide semiconductor field-effect transistor (MOSFET) having a structure called a diode-connected structure in which the gate and drain thereof are shorted. Incidentally, only a voltage equal to the threshold voltage of the transistor is applied as the gate voltage.
However, if the voltage difference between the photogenerating cell 30 and storage cell 2 is so large that the charging current increases, the supply of current to the backflow preventing diode 40 must be increased. Therefore, the backflow preventing diode 40 is structured so that the ratio of a gate width to a gate length relevant to the diode-connected MOSFET will be large.
When the backflow preventing diode 40 is adopted, if the voltage difference between the photogenerating cell 30 and storage cell 2 is small (approximately 0.4 V or less) or if electromotive force is limited because no light falls on the photogenerating cell 30 (low intensity of illumination), charging is not achieved efficiently. Moreover, the area of the backflow preventing diode 40 must be increased in order to ensure a sufficient supply of current. This leads to an increase in the area of a system LSI in which the charging circuit 6 is incorporated.
As a means for solving the above problems, a method according to which an operational amplifier is used to sense a voltage difference between two different power supplies for the purpose of logically switching between charging and non-charging has been disclosed in U.S. Pat. No. 4,291,266.
However, according to the method, a storage cell to be charged is used to drive the operational amplifier. The operational amplifier is therefore driven during non-charging. Consequently, the energy of the storage cell is consumed. This poses a problem especially when super-low power is used for driving.